1. Field of the Invention
The present invention relates to a method and a system of managing wafers in a semiconductor device-producing facility and, more particularly, to automatically tracking, identifying and sorting the wafers.
2. Description of the Related Art
The manufacture of semiconductor devices requires a number of discrete process steps to create a packaged semiconductor circuit device from a semiconductor substrate, which is usually provided as a semiconductor wafer. The semiconductor manufacturer fabricates semiconductor circuit devices, e.g., microprocessors, DRAMs, ASICs (application specific ICs) on individual wafers, usually forming a number of devices on each wafer. The individual fabrication processes include photolithography, ion implantation, etching, deposition, and other associated fabrication processes known in the art.
Typically, selected semiconductor devices formed on a specified wafer, herein referred to as a measurement wafer, are used for measuring device characteristics during and after the fabrication process, thereby monitoring the quality of the manufacturing process. Moreover, test wafers are introduced into the various fabrication processes to control and properly adjust process parameters of these steps. In addition, further types of wafers are generally used, referred to as dummy wafers, which are required to balance the number of wafers in a specific process step when a varying number of wafers to be processed would otherwise lead to varying process constraints in the process step.
Another important factor in manufacturing high quality semiconductor devices with associated large production yields is to avoid contamination of the wafers to be processed with any type of particles. As the human being is one of the most significant sources of contaminating particles, the entire manufacturing process takes place in a clean room in an attempt to reduce the number of environmental particles. Moreover, all procedural steps, including the provision of the wafers to the various process stations, is, as far as possible, automated, thereby reducing the presence of a human operator to a minimum.
In present semiconductor fabrication facilities, computer-aided manufacturing systems (CAM systems) control the fabrication process and provide information regarding operating conditions during these processes. Generally, the wafers undergo a specific treatment or process flow, i.e., the entirety of procedural steps necessary for forming a specific semiconductor device such as applying photoresist, irradiating the wafer with exposure light, developing the resist and etching the wafer, etc., to produce the desired device on the wafer. The wafers are generally stored in a single wafer cassette and are routed through the corresponding process stations to perform the desired process steps. One or more wafers to be subjected to a specific treatment form a specific lot of wafers. When the specific treatment is completed, a control unit, which may be implemented in a CAM system, instructs an operator or an automated transport system to transport the corresponding wafer cassette containing said lot of wafers, or at least a part of said wafer lot, to a new starting point for another process sequence or operation.
In order to maintain high product quality, by continuously controlling the efficiency of the various processes, it is inevitable to add dummy or test wafers to a specific lot of wafers. Since present CAM systems do not distinguish single wafers, but merely track individual wafer cassettes and wafer lots, respectively, it is not possible to automatically constitute specific test-wafer assemblies and measurement wafer assemblies, wherein single test wafers or measurement wafers of different wafer lots are finally grouped together in order to obtain the desired information regarding the various process steps. Moreover, each process sequence for manufacturing a certain semiconductor device is assigned to an individual wafer lot contained in a single cassette, i.e., even if a single wafer has to be subjected to a specific process sequence, an entire wafer cassette has to be employed, thereby wasting the remaining space of the wafer cassette and thus leading to a reduced throughput if a plurality of small wafer lots has to be processed.
Since the demand for ASICs has recently increased, the semiconductor manufacturers have to deal with a large variety of different devices and, accordingly, with different processes, wherein the number of pieces is often small, the number of wafers manufactured per lot may accordingly be relatively low. As a consequence, a large amount of the production capacity of the semiconductor manufacturing facility is wasted, owing to the limited ability of a conventional CAM system to control the wafer flow through the facility merely on a single cassette basis. Furthermore, the presence of wafers to be processed according to differing design demands requires the existence of a variety of test and dummy wafer assemblies within the production line which, in turn, need to be processed in separate cassettes, thereby additionally reducing the throughput of product wafers. Similarly, manufacturing of semiconductor devices using cutting edge technology requires extensive testing and controlling of test wafers as well as of a portion of the product wafers, i.e., the measurement wafers, in order to achieve a high product yield in combination with a predetermined quality standard. In the conventional CAM system, however, the flexible and automated handling of product wafers and test wafers and monitoring test results on a single wafer basis is not possible.
The present invention is directed to a method of making a semiconductor device that solves, or at least reduces, some or all of the aforementioned problems.